It is well accepted that bioresorbable orthopedic implants are always the better choice than permanent foreign-body implants, as long as their bioresorption rates, biomechanical properties and variations in biomechanical properties with respect to the resorption processes are appropriately controlled. Among all bioresorbable orthopedic implants, calcium-based implants (calcium phosphate, calcium sulfate, etc), are perhaps the top choice so far.
For the purpose of filling a bone cavity, especially an irregularly-shaped bone cavity, a bone cement paste (for example, a PMMA, calcium phosphate cement or calcium sulfate cement) is often injected into the cavity, wherein the bone cement paste is hardened in-situ. This hardened cement will remain in bone as a permanent implant if it is a permanent foreign-body implant such as PMMA, or gradually replaced by natural bone if it is a bioresorbable material such as calcium phosphate or calcium sulfate. For load-bearing applications, this hardened cement should provide a sufficient strength to withstand the post-operation routine loadings.
Most conventional methods of forming a hardened (set) bone cement in bone cavity involve creating a bone cavity, followed by directly injecting a cement paste into the bone cavity. Such an approach suffers the following major drawbacks among others:    (1) Since the cement paste is directly injected into an environment filled with blood/body fluid, the cement particles are easily dispersed in this environment, especially before the paste is fully set. The dispersed cement particles can penetrate into surrounding tissues, cracks, blood vessels, nerve system, etc. and cause various kinds of clinical complications such as potentially fatal cement embolism.    (2) Since the cement paste is hardened in blood/body fluid, the predetermined liquid/powder ratio, which is critical to cement properties, is disrupted in-situ, causing the performance/properties of the cement to degrade. Although applying pressure to the cement during its hardening process can improve the cement strength, surgeons usually avoid applying a high pressure directly to the injected cement paste due to the above-mentioned potential risks of complications.    (3) Besides the disruption in liquid/powder ratio, the irregular shape of the hardened cement also decreases the biomechanical properties of the cement and increases the uncertainty/risks of the cement performance (depending on the actual shape and filling condition), especially for bioceramic cements such as calcium phosphate cement and calcium sulfate cement. The decreased strength further causes the cement to more easily disperse/disintegrate.
Another approach to inject an orthopedic implant into a bone cavity involves inserting a container (balloon or pocket) into the cavity; injecting a bone filler (not necessarily a hardenable cement paste) into the container through a tube; and separating the container from the tube with the container and its contained bone filler remaining in bone. One major problem with this approach is that the container left in bone becomes a permanent foreign body which prevents the bone filler from directly interacting with bone tissue to form a biological or even only a chemical or physical bond between the bone filler and bone. Furthermore, most popularly-used containers (balloons) are made from polymers which are not bioactive, bioconductive, or even biocompatible. The negative effects of this permanently implanted container are most obvious when the bone filler is a bioresorbable material, such as a calcium phosphate or calcium sulfate-based material. In this case even a biodegradable polymer container hinders the bioresorption process of the bioresorbable bone filler for a season, especially during the most critical early stage resorption/healing process. Furthermore, most biodegradable polymers do not demonstrate mechanical properties as desired.
An improved method for forming a hardened cement in a bone cavity involves inserting an inflatable, preferably inflatable and expandable, pocket into a bone being treated; injecting a hardenable cement paste into the pocket through a tube which connects and carries the pocket into the bone; allowing the cement paste to harden within the pocket in the bone cavity; opening the pocket; separating the pocket from the hardened cement, and retrieving the opened pocket from the bone with the hardened cement remaining in the bone. Advantages of this method include allowing the hardened cement implant to directly contact the surrounding bone tissue thus enhancing the healing process, and the much higher strength of the hardened cement compared to that of the cement paste directly injected into the bone cavity. This is especially advantageous for bioresorbable implants. A typical example can be found in U.S. Pat. No. 7,306,610 B2.
A further improved method for forming a hardened cement in a bone cavity involves inserting an inflatable, preferably inflatable and expandable, pocket into a bone being treated; injecting a hardenable cement paste into the pocket through a tube which connects and carries the pocket into the bone, therein said pocket is made from a material penetrable to liquid but substantially impenetrable to the powder of said cement paste; allowing the cement paste to harden within the pocket in the bone cavity; opening the pocket; separating the pocket from the hardened cement, and retrieving the opened pocket from the bone with the hardened cement remaining in the bone. A primary advantage of this method is allowing a portion of the liquid contained in the cement paste to be expelled out of the pocket, especially when a pressure is applied unto said cement paste before said cement paste is substantially hardened, so that the powder/liquid ratio of said cement paste in said pocket is increased and the strength of the hardened cement is further increased. This further increase in cement strength is especially advantageous for the relatively weak ceramic, calcium-based cement. A typical example can be found in U.S. Pat. No. 7,144,398 B2. Nevertheless, one major difficulty in practicing this method is the accurate control of the selective penetrability (only to liquid) of the pocket, especially during the expansion process, wherein the volume of the pocket continues to increase while the thickness of the pocket continues to decrease.